Spaceborne remote sensing instruments (e.g. satellite altimeters) are often employed for measuring oceanographic phenomena, such as sea surface elevation, near surface ocean wind speed and wave height. Although an on-board (transmit/receive) radar altimeter is capable of performing distance/height measurements, such measurements are susceptible to atmospheric variations, such as irregularities in the water vapor content in the atmosphere, which affect the propagation delay of the altimeter's radar pulse. To accommodate such variations, the altimeter-derived measurements must be adjusted or `corrected.`
Currently, there are essentially two mechanisms for providing the necessary modification of the radar return. The first involves correcting the altimeter data based upon a prescribed correction chart or look-up table of adjustment parameters. This method is coarse, however, since it is usually based upon a previously derived average value for the geographical location of interest. A second approach is employ a separate radiometer to measure (in real time) the geophysical parameter of interest (e.g atmospheric water vapor, rain, atmospheric liquid water). Because the principal performance requirement of a radiometer antenna is beam efficiency, while that of the radar altimeter antenna is high gain, each of the two subsystems has its own antenna, with the look aperture of the radiometer antenna being directed off-nadir to a location on the ocean surface that is non-coincident with the measuring location of the radar antenna.
Because of this non-coincidence of the radar antenna and radiometer antenna apertures, the data derived from the radiometer must be subjected to a geo-location algorithm in the course of using the radiometer data to adjust radar altimeter measurements. Thus, the second approach not only increases the amount of spaceborne hardware, but it adds to the computational intensity of the overall distance measurement process and adds measurement accuracy errors.